This invention relates to an exhaust system for an internal combustion engine and more particularly relates to an exhaust system for marine engines.
In-board mounted marine engines conventionally are provided with some means of reducing the sound levels associated with the operation of internal combustion engines. With the increased popularity of boating, it has become even more important to reduce the noise levels attendant to the operation of water craft. Unnecessary noise detracts from the pleasure of boating and can be an annoyance to the boat occupants and others in the area. Further, regulations imposed by various local, state and federal agencies are becoming more stringent in establishing maximum acceptable noise levels.
In an effort to comply with these regulations and requirements various exhaust silencing systems have been developed. Conventional exhaust mufflers can be installed in the exhaust pipe of the boat engine. Exhaust mufflers to be efficient as sound silencers, must decrease exhaust gas velocity and absorb the sound waves or cancel them by interference with other waves from the same source. Mufflers typically should have a volume of approximately 6 to 8 times the engine displacement and may further contain baffles to reduce the velocity across the muffler. In such systems exhaut back pressure should be kept to a minimum since increased back pressure will have a detrimental effect on the maximum power output of the engine and will cause increased engine wear.
It has also been suggested in the prior art to provide for the escape of exhaust gases from boats beneath the water level. The water serves to decrease exhaust gas velocity and absorbs the sound waves operating as an effective muffler. Most systems of this type terminate the exhaust pipe from the engine at a location beneath the boat so that in operation exhaust gases are discharged below the water level. By exhausting escaping gases beneath the boat the sound of exhaust is effectively muffled and gases are prevented from escaping into the air to the annoyance of the occupants of the boat. It is also known to incorporate a check valve at the terminal end of the exhaust pipe to prevent water from entering into the exhaust system. The pressure of the exhaust gases serves to open the check valves when the engine is started. Systems of this general type are found to be effective, however, they do not always reduce noise levels to an acceptable minimum. Further, systems of this type, particularly those including a check valve, may result in unacceptable increases and back pressure therefore reduce the power output of the engine. Performance may be particularly adversely effected when the boat engine is idling and the boat is not underway but is still in the water. In this condition back pressure can impose considerable load on the engine.
It has also been suggested in the prior art to incorporate a directional exhaust flow regulator in boats having internal combustion engines. The regulator conducts exhaust gases to the atmosphere when the boat is not moving and directs the exhaust gases beneath the water when the boat is underway. Such devices while effective, involve complicated valving arrangements requiring frequent maintenance and which are expensive in initial first cost.